The present invention relates to sensing an object with a plurality of conductors. Some embodiments relate to touch sensitive pads that sense a human finger or a stylus.
Touch sensitive pads are used in computers, microwave ovens, and other devices. A touch sensitive pad allows a user (a human being) to enter information by touching the pad with a finger or a stylus. In computer systems, touch sensitive pads provide a compact alternative to a mouse, and are used extensively in laptop and notebook computers. Some systems allow the user to enter verbal information into a computer by writing the information on a touch sensitive pad. Some devices do not require the user to touch the pad, it is sufficient for the user to place the object (the finger or the stylus) close to the pad. We will still call these pads “touch sensitive” herein.
FIG. 1 illustrates an exemplary system with a touch sensitive pad 110. The pad has a number of conductors 120X, 120Y. Conductors 120X are parallel to each other, and conductors 120Y are parallel to each other and perpendicular to conductors 120X. A human finger 130 touches the pad and thereby changes the state of some or all of conductors 120 (i.e. 120X, 120Y). For example, the presence or pressure of finger 130 may change the capacitance of the conductors passing under the finger. Sensing and processing circuit 140 senses the conductors' states and determines the position and, possibly, the pressure of finger 130 on the pad. Circuit 140 then performs an appropriate action, e.g. moving a cursor on a computer screen, turning on a microwave oven, and so on.
Conductors 120 do not have to be arranged as two groups of parallel conductors, but can be arranged radially or in some other fashion.
FIG. 2 shows a sensing and processing circuit 140 of the type disclosed in U.S. Pat. No. 4,736,191 issued Apr. 5, 1988 to Matzke et al. Conductors 120, shown as X1, . . . XN, are connected through appropriate resistors to inputs of multiplexer 210. Multiplexer 210 selects one conductor at a time, and connects the selected conductor to a line 214. Processing circuit 220 generates a signal indicative of the capacitance on line 214 and hence the capacitance of the selected conductor. The signal is generated as follows. Line 214 is discharged to ground, and then connected to a high voltage provided by microprocessor 230. Circuit 220 generates a pulse indicative of a period of time in which the voltage on line 214 rises to some predetermined value. Microprocessor 230 measures the length of this pulse and stores it as an indication of the capacitance of the selected conductor. After suitable processing, the microprocessor causes multiplexer 210 to select the next conductor, and so on.
The conductors are thus processed sequentially, as indicated in the timing diagram of FIG. 3. TX1 is the time of processing the conductor X1, TX2 is the time of processing the conductor X2, and so on.
FIG. 4 illustrates a system in which the conductors are processed simultaneously. Each conductor 120 is connected to a corresponding charge integrator 410. Each charge integrator 410 generates a voltage indicative of the respective conductor's capacitance, and provides the voltage to a respective circuit 420. Circuits 420 perform sample-and-hold and filtering functions. The outputs of circuits 420 are connected to analog to digital converter 430. See U.S. Pat. No. 5,914,465 issued Jun. 22, 1999 to Allen et al.
As illustrated in the timing diagram of FIG. 5, all of the conductors are processed simultaneously.